Continuous hydrogenation of compounds may take place in a solvent or mixture of solvents in the presence of a heterogeneous catalyst and hydrogen under elevated pressure. Typically, the hydrogenated compounds are then separated from at least part of the hydrogenation solution and the remaining solution is returned to the reaction. Depending on the design of the hydrogenation reactor and the reaction conditions, a hydrogen-containing waste gas is formed during this process. To increase economic efficiency, it is desirable to compress at least part of this waste hydrogenation gas and return it to the reaction.
A generic hydrogenation process which can be operated on an industrial scale is the anthraquinone process for the production of hydrogen peroxide (see, Ullmann's Encyclopedia of Industrial Chemistry, vol. A13:447–456 (1989)). In this, a working solution is formed by dissolving one or more reaction supports (anthraquinone derivatives and/or tetrahydroanthraquinone derivatives) in an organic solvent or mixture of solvents. Hydrogenation then takes place in the presence of a suspension or fixed bed catalyst. During the hydrogenation step, at least a portion of the derivatives is converted to the corresponding anthrahydroquinone derivative(s) or tetrahydroanthrahydroquinone derivative(s). Hydrogenated working solution freed of catalyst is gassed in a subsequent oxidation step with oxygen or an oxygen-containing gas, usually air, to convert the reaction support back into the quinone form with the formation of hydrogen peroxide. The hydrogen peroxide formed is extracted from this “oxidized working solution” using water or a dilute aqueous hydrogen peroxide solution. The “extracted working solution” thus obtained (which contains the reaction support or supports in oxidised form) is returned to the hydrogenation reaction. In practice, only part of the hydrogenated working solution is fed into the oxidation step; the greater part is returned directly into the hydrogenation step (hydrogenation circuit).
EP-B 0 111 133 describes a anthraquinone process in which hydrogenation takes place in a loop reactor in such a way that hydrogen reacts substantially completely and essentially only inert gases are flushed out from a pump supply vessel, which simultaneously acts as a gas separator. In order to favour hydrogenation kinetics, however, it can also be advantageous to work in such a way that the waste hydrogenation gas contains hydrogen. In general, if there is a small quantity of hydrogen in the waste hydrogenation gas, it is preferably burnt. If there is a larger quantity of hydrogen in the waste hydrogenation gas, then it is preferable to feed this back to the reaction. In addition, in tubular reactors as well as other hydrogenation reactors, e.g., gas-lift reactors or fixed bed reactors, situations can arise where hydrogen or a hydrogen/inert gas mixture has to be recycled.
In the anthraquinone process of Laporte Chemicals (Chemical and Process Engineering, 01/1959, 5–6 and 452–453 of the Ullmann document cited above) hydrogenation takes place in the presence of a suspension catalyst in a gas-lift reactor. Since the hydrogen does not react completely on one pass through the reactor, it is used in excess and the waste hydrogenation gas is compressed and recycled together with fresh hydrogen.
Mechanical gas compressors, e.g. compressors working on the fluid principle, have generally been used to compress hydrogen-containing waste hydrogenation gas drawn off from a separator downstream of the reactor or from the reactor chamber itself. These compressors are electrically driven, consume a large amount of energy and require high-maintenance. To avoid these disadvantages, steam operated injectors have sometimes been used for the intake and compression of waste hydrogenation gas (see EP patent 0 812 297). One disadvantage of these injectors is that they can only be used economically in a location where excess steam is available, e.g., in a paper factory. Another disadvantage of is that the waste hydrogenation gas compressed has to be separated from the steam condensate formed before it can be fed into the hydrogenation reactor. If there is insufficient separation, an undesirably large amount of water can pass into the reactor, resulting in a reduction in the hydrogenation activity of the suspension catalyst.